1. Field of the Invention
The present invention relates to a vehicle speed control system for controlling vehicle speeds by controlling either or both of the displacement of the hydraulic pump and hydraulic motor in the hydraulic transmission.
2. Description of the Prior Art
A conventional hydraulic transmission typically has a variable capacity hydraulic pump (e.g. swash plate type pump) to be driven by an engine on the engine side and a variable capacity hydraulic motor (swash plate type motor) on the driving shaft side for driving vehicle wheels or a crawler belt, and transmits the driving force of the engine to the wheels or the crawler belt by directing the operating fluid discharged from the pump through the hydraulic piping. Speed control of the vehicle is performed by adjusting the discharge amount of the hydraulic pump and the suction amount of the hydraulic motor per rotation of the engine by means of controlling the slope angle of both the swash plates of the hydraulic pump and the hydraulic motor, thereby changing the speed of the driving shaft for each rotation of the engine.
Since the conventional vehicle speed control system is designed to perform vehicle speed control by mechanically controlling the slope angle of the swash plate by the movement caused by the operation of a vehicle speed setting lever, a brake lever and a throttle lever by the vehicle operator, the structure of the control system becomes unavoidably complex. Accordingly, in the conventional system performing vehicle speed control is difficult and therefore driving comfort, safety and other factors are likely to be neglected.